1. Field of the Invention
The present invention relates to a method for producing perfluoroethane. More particularly, the present invention relates to a method for producing perfluoroethane from ethylenic hydrofluorocarbon in the presence of a cobalt catalyst. The term "ethylenic hydrofluorocarbon" as used herein means a fluoroethane containing 1-5 hydrogen atoms, represented by C.sub.2 F.sub.x H.sub.y 1.ltoreq.x, 1.ltoreq.y.ltoreq.5, x+y=6.
2. Description of the Related Art
Forming a gas phase at room temperature, perfluoroethane (hexafluoroethane) with a boiling point of -79 to -78.6.degree. C. is a very useful industrial compound. For example, it is used as an etching gas for silicon wafers in the manufacture of semiconductors, a cleaning gas, an electrically insulating gas, a leak test gas, etc. In particular, this gaseous compound is required to be extremely low in its impurity content when used for the manufacture of semiconductor devices.
There are various known methods for manufacturing perfluoroethane, which are typically divided into five types as follows:
(1) Direct fluorination: ethane gas is directly reacted with fluorine gas (F.sub.2);
(2) Electrochemical fluorination: ethane or ethylene is fluorinated under an electrolysis condition;
(3) Hydrofluorination: perhaloethane (C.sub.2 F.sub.x Cl.sub.y) compounds are fluorinated in the presence of a catalyst;
(4) Pyrolytic fluorination: tetrafluoroethylene is fluorinated while being thermally decomposed with CO.sub.2 ; and
(5) compounds with triple bonds, such as acetylene, are reacted with metal fluoride, such as CoF.sub.3, MnF.sub.3, AgF.sub.2, etc., for fluorination (Japanese Pat. Laid-Open Publication Nos. Heisei 3-167,141).
The direct fluorination in which the hydrogen atoms of ethane are directly replaced with fluorine gas (F.sub.2) is conducted under extreme conditions showing a heat of reaction at 102-104 kcal/mole, so that cleavage occurs at the C--C bond as well as the C--H bonds, producing a large quantity of CF.sub.4. In order to reduce the C--C bond cleavage, fluorine gas is diluted with an inert gas such as nitrogen gas before participation in the fluorination. Such an inert gas certainly alleviates the cleaving of the intercarboneous bond, but causes a significant problem particularly when the perfluoroethane gas is used in the fabrication of semiconductor devices because it may act as an impurity. In addition, low boiling point compounds like perfluoroethane (Bp: -79 to -78.6.degree. C.) have difficulty separating from nitrogen gas, causing significant loss upon the purification. Further, the direct feeding of fluorine gas requires a reactor of a special structure, which gives rise to corrosion in the apparatus, and produces difficulty in temperature control on account of the explosive reaction. Therefore, the direct fluorination is disadvantageous in its application for the commercialization of the production of such a low boiling point compounds. On the other hand, it is recommended to be applied for the production of such compound that are relatively high in boiling point with large molecular weights because they can be easily separable from the inert gas.
In accordance with the electrochemical fluorination, ethane is fed below a specially constructed carbon electrode in a fluorine gas-electrolyzing bath to react to the fluorine gas, which is generated along the surface of the carbon electrode. This method, however, produces a large quantity of by-products like the direct fluorination. In addition, because the electrochemical fluorination occurs on the surface of the electrode, the feedstock cannot help being provided at a limited amount in connection to the limited length of the electrode. So, the electrochemical fluorination suffers from a significant problem of being poor in production yield.
As for the hydrofluorination, it is mainly applied for the reaction of perhaloethanes such as trichlorotrifluoroethane (CFC-113), dichloro trifluoroethane (CFC-114) and chloropentafluoroethane (CFC-115) with hydrogen fluoride in the presence of a catalyst. For this catalytic reaction, Cr or Al catalysts are used at high temperatures under high pressures. In the perhaloethane, e.g., C.sub.2 F.sub.5 Cl, just before complete fluorination, the bond between the carbon and the last chlorine atom is very strong owing to the influence (electronegativity) of adjacent fluorine atoms, so that the chlorine atom is very difficult to replace with a fluorine atom. In other words, the hydrofluorination shows low conversion rates into perfluoroethane. High temperatures and high pressures are thus needed to overcome such low conversion rates, but require a high cost of equipment.
When perfluoroethane is prepared from tetrafluoroethylene, this reactant is directly reacted with fluorine gas or thermally decomposed by use of CO.sub.2. The direct reaction of C.sub.2 F.sub.4 with F.sub.2 gas is very volatile, producing a large quantity of CF.sub.4, like the above direct fluorination. The preparation of perfluoroethane through the thermal decomposition reaction between C.sub.2 F.sub.4 and CO.sub.2 demands high temperatures of greater than 700.degree. C. and is problematic in that the restraint of CF.sub.4 production must rely on temperature control.
Japanese Pat. Laid-Open Publication No. Heisei 3-167141 uses compounds with intramolecular triple bonds, such as acetylene, methylacetylene, 2-butyne, methylethylacethylene, etc, as materials for preparation of perfluoroethane, disclosing that such a triple bond compound is converted into perfluoroethane in the presence of a CoF.sub.3 catalyst with a minimum of C--C bond cleavage. (CF.sub.4 production 0.2%) based on the following reaction formulas: EQU C.sub.2 H.sub.2 +8CoF.sub.3 .fwdarw.C.sub.2 F.sub.6 +2HF+8CoF.sub.2 EQU 8CoF.sub.2 +4F.sub.2 .fwdarw.8CoF.sub.3
The patent lays emphasis on the high selectivity for perfluoroethane (97% or greater), reporting the loss of two HF equivalents only. However, a significant disadvantage of this reaction is the extreme preponderance of the catalyst over the reactant used. For instance, the amount of the reactant which can be fed in the reaction is merely 5% of that of the catalyst fed (catalyst 55 kg, reactant 800 ml/min, reaction period 2 hrs). The reason is that 8 moles of CoF3 are consumed per mole of C.sub.2 H.sub.2 as shown in the above reaction formulas. Where the feedstock C.sub.2 H.sub.2 is used greater than 5% of the amount of the catalyst, it is virtually impossible for all of the feedstock to advance to the final product C.sub.2 F.sub.6 and it seems to produce a large quantity of the intermediate product. This coincides with the disclosure of another document (Russia Journal of Organic Chemistry Vol. 30, No. 8 (1994)). A plant which utilizes this reaction would have a reactor whose size is exceptionally large. Thus, the method has disadvantages of being economically unfavorable and poor in conversion rate.
The fluorination of triple bond-containing compounds such as acetylene into perfluoroethane can be easily achieved. But the feedstock is expensive and also dangerous requiring careful treatment.